Storage and reproduction method and apparatus

ABSTRACT

Digital audio recording equipment is used to record modulated signals, such as the RF test signals generated by test modems. If the modulated signals are outside the audio frequency range, they are down-converted into the audio frequency range using a mixer, local oscillator and a filter. The digitally converted data are recorded onto a suitable medium, such as a DAT, CD or a computer memory. The data is then disseminated for reproduction by a suitable digital audio player. The reproduced audio signal may be up-converted to the desired radio frequency using a mixer, local oscillator and a filter. The reproduced signal may be used to test demodulators, codecs or transceivers. The signals output by the transceiver and the reproduced signal may be recorded on different channels of the digital audio recording equipment. The recording equipment may be used to capture RF signals for later analysis. The player may be used for reproducing modulated signals for broadcast.

TECHNICAL FIELD

The present invention relates to a method and apparatus for storageand/or reproduction of modulated signals, particularly but notexclusively in apparatus for testing radio equipment.

BACKGROUND ART

In one conventional method of testing radio equipment, a radiotransmitter is connected directly to the equipment to be tested and theradio transmitter generates test signals. Typically, the transmitter isconnected to the equipment to be tested via a channel simulator, whichsimulates effects such as fading, Doppler shift and interference. Anexample of an RF channel simulator is described in GB-A-2283392. Thesefunctions may be built into the transmitter.

This conventional method requires the transmitter and channel simulationequipment to be transported to the test site. Typically, the transmitterand channel simulator are expensive equipment and it is therefore verycostly to provide multiple sets of such signal generation equipment ifmore than one party needs to test their equipment at the same time.

In an alternative conventional method, test signals are broadcast overthe air, so that the signal generation equipment does not need to betransported to the receiving party, and multiple receiving parties cantest their receiving equipment at the same time. However, this method isnot entirely satisfactory, because the signal is degraded between thetransmitting and the receiving party in an unpredictable manner, so thatthe desired characteristics of the test signal may be lost.

Moreover, broadcasting such test signals over the air involves the riskof disclosing confidential information concerning signal characteristicsto third parties.

DISCLOSURE OF THE INVENTION

According to one aspect of the present invention, there is provided amethod and apparatus for storing radio frequency signals, in which anarrow band of the radio frequency spectrum, having an audio frequencybandwidth, is down converted into the audio frequency range and recordedon an audio-frequency storage medium so that the amplitude, phase andfrequency characteristics of the RF spectrum are preserved.

According to another aspect of the present invention, there is provideda method and apparatus for generating radio frequency signals, in whichan audio-frequency signal is reproduced from the audio-frequency digitalstorage medium and is converted into a narrow band of the radiofrequency spectrum so as to reproduce the amplitude, phase and frequencycharacteristics of a radio frequency signal.

In this way, radio frequency signals can be stored and reproducedaccurately using inexpensive and commonly available digital audiostorage media, such as digital audio tapes, optical or magneto-opticaldiscs or computers having audio processing facilities.

Advantageously, such a method and apparatus can be used to provide testsignals for testing the performance of a demodulator and/or a codec.Alternatively, a two-channel digital audio recorder may be used torecord RF signals received by a device to be tested on one channel, andthe signals generated by the device under test in response to the testsignals, on the other channel. Thus, a two-way protocol exchange can bestored conveniently for later analysis.

In another advantageous embodiment, the digital audio recorder may beused to record burst signals for analysis in non-real time. Therecording of burst signals may be triggered by the detection of suchburst signals.

In another advantageous embodiment, the reproduced test signals aresimultaneously supplied to multiple demodulators or signal analysers,for teaching purposes.

In another advantageous embodiment, multiple copies of the recordedsignal are distributed to local broadcasting stations for subsequentbroadcast. In this way, broadcast programmes can be disseminated in aconvenient form, without requiring complex modulating equipment at thebroadcast station.

BRIEF DESCRIPTION OF THE DRAWINGS

Specific embodiments of the present invention will now be described withreference to the accompanying drawings in which:

FIG. 1 is a schematic diagram of apparatus for recording radio frequencysignals in a first embodiment of the present invention;

FIG. 2 is a diagram showing the frequency response of an audio frequencyrecorder and the amplitude of signals down-converted from a narrow bandof the RF spectrum into the audio frequency spectrum;

FIG. 3 is a diagram of apparatus for reproducing RF signals from adigital audio storage medium in a second embodiment of the presentinvention;

FIG. 4 is a diagram showing a third embodiment including the embodimentof FIG. 3;

FIG. 5 is a diagram showing a fourth embodiment including theembodiments of FIGS. 1 and 3;

FIG. 6 is a diagram showing a fifth embodiment including the embodimentof FIG. 1;

FIG. 7 is a diagram showing a sixth embodiment including the embodimentof FIG. 3;

FIG. 8 is a diagram showing a seventh embodiment including theembodiment of FIG. 3;

FIG. 9 is a block diagram of an exemplary embodiment of a recorder; and

FIG. 10 is a block diagram of an exemplary embodiment of a player.

MODES FOR CARRYING OUT THE INVENTION

Apparatus for recording a narrow band of the RF spectrum on a digitalaudio recorder will now be described in detail with reference to FIGS. 1and 2. Radio frequency signals RF are supplied on a line 2. Line 2 maybe connected to a receiving antenna, to an RF signal generator or to atest modem. In one example, the line 2 is connected to an Inmarsat-M™modem which generates signals complying with the Inmarsat-M™ signalparameters, at around 70 MHz. The Inmarsat -M satellite system isdescribed in more detail in “Satellite Communications:Principles andApplications” by Calcutt & Tetley, First Edition, 1994, published byEdward Arnold.

The RF signals are input to a mixer 4, where they are multiplied withthe output of a local oscillator 6. The local oscillator 6 generates asingle frequency. In one example, the local oscillator 6 is a HewlettPackard 8648B generating an output signal at 70 MHz, although a simpleroscillator such as a crystal oscillator may be used provided it cangenerate stable signals in the radio frequency range of interest.

The output of the mixer 4 comprises signals of frequency R+LO and RF−LO,where RF is the radio frequency and LO is the local oscillator outputfrequency. For example, if the frequency of the R signal is 70.015 MHzand the local oscillator output frequency is 70.000 MHz, the output ofthe mixer 4 will comprise signals of frequencies 140.015 MHz (RF+LO) and15 kHz (RF−LO).

If another signal is present at 69.085 MHz, this signal will generate an“image” signal, also at 15 kHz. This image signal may be removed bywell-known techniques, such as two-stage down-conversion with theundesired frequencies being filtered out after the first stage. If theRF signals are output from a test modem, such signals at less than theoscillator frequency LO may not be generated.

Alternatively, the “image” signal of lower frequency than the oscillatorfrequency LO may be passed for recording and the signal having a higherfrequency than the oscillator frequency may be filtered out. The “image”signal has a reversed spectrum, but this may be compensated for onplayback, as described below.

The output signal of the mixer 4 is passed through a filter 8 whichpasses the frequency RF−LO but removes the frequency RF+LO. For example,the filter may be a low-pass filter such as a Mini Circuits SLP-1.9. Thesignal RF−LO is output from the filter 8 to a variable gain amplifier 10or a fixed gain amplifier of suitable gain, which controls the level ofthe signal input to a recorder 12. In one example, the amplifier is aMini Circuits ZFL-500 amplifier.

“The recorder has at least one analog input which receives the output ofthe variable amplifier 10. The recorder 12 includes an A/D converter andmeans for storing the output of the A/D converter. FIG. 9 is a blockdiagram of an exemplary recorder 12 showing the A/D converter 13 and themeans for storing 15 to store digital signals.

A number of different types of digital audio recorder are currentlywidely available at low cost. In one specific example, the recorder 12was a SONY™ PCM 2600 digital audio tape recorder. The frequency responseof such a recorder, as shown by the dotted line in the graph of FIG. 2,is substantially flat between 20 Hz and 21 kHz, which corresponds to theaudio frequency range. Alternatively, the recorder 12 may be a personalcomputer, such as a PC-compatible or Macintosh™ computer, fitted with anaudio card having an analog input and an A/D converter. The digitallyconverted signal may be stored in the main memory of the computer, andmay additionally be stored on a non-volatile storage medium, such as ahard disc or recordable CD.

The filter 8 can be dispensed with if the variable amplifier 10 has afrequency response which is sufficiently low to filter out the frequencyRF+LO, or if the response characteristic of the recorder 12 excludes thefrequency RF+LO. However, the application of high-frequency signals tothe recorder 12 may cause interference in the electronic circuitry ofthe recorder, in which case the presence of the filter 8 isadvantageous.

FIG. 2 shows, together with the frequency response characteristic of therecorder 12, an example of the frequency spectrum of the down-convertedsignal RF−LO. The signal RF contains signalling in two channels C1 andC2 of 5 kHz bandwidth, at 70.010 and 70.015 MHz respectively. A noisesignal N is also present and interferes with the first channel C1. Thesignal RF is down-converted so that the channel C1 is recorded at 10 kHzand the second channel C2 is recorded at 15 kHz. It can therefore beseen that the device shown in FIG. 1 is capable of recording accuratelythe characteristics of a narrow-band RF spectrum, such as modulationcharacteristics, fading, frequency drift and interference. It is forthis reason that a digital audio recorder of sufficiently high fidelityshould be used, in order to preserve and reproduce the amplitude phaseand frequency characteristics of the RF spectrum.

Certain types of digital audio recorder which compress audio signalsusing an algorithm optimized for music or speech, such as the SONYminidisc™ recorder, may not be suitable for this purpose.

An apparatus for reproducing the recorded RF signal will now bedescribed with reference to FIG. 3. A digital audio player 12′ may beused to clay back the digital audio signals stored by the recorder 12.The player 12′ is compatible with the recorder 12 with which the signalwas recorded, and may comprise similar or the same equipment as therecorder 12. The player 12′ may play back a digital audio tape recordedby the recorder 12 or may reproduce the audio signal from a CD or harddisc on which the digital audio data were recorded by the recorder 12.The CD may alternatively be a pressing taken from a master recording bythe recorder 12. Alternatively, the digital audio data CD may be sent asa file of suitable format, such as a wave format, to the computerforming the player 12′. The audio file may be sent between the computersthrough a communications link of any suitable type.

Alternatively, the digital audio data may be synthesized by a suitablyprogrammed computer so that, when reproduced, the digital audio datagenerate an RF signal having the desired characteristics. Thesynthesized digital audio data may be stored on any of the storage mediadescribed above, for reproduction by the player 12′.

“FIG. 10 is a block diagram of an exemplary embodiment of a player 12′having a digital/analog converter 13′ and a means for storing 15 tostore digital signals.”

The player 12′ converts the digital audio data to an analog audiointermediate frequency signal IF, on line 14, which is connected to amixer 16. A further local oscillator 18 outputs an oscillator signal offrequency LO′ which may correspond to the frequency of the output of thelocal oscillator 6 or may be a different frequency, to another input ofthe mixer 16. The mixer 16 multiplies the two input signals together andoutputs signals of frequency LO′+IF and LO′−IF. In one example, theintermediate frequency IF is 20 kHz and the local oscillator frequencyLO′ is 70 MHz, so that the output of the mixer 16 comprises signals offrequency 70.02 MHz and 69.98 MHz. The output of the mixer 16 is passedthrough a band-pass filter 20, which removes the component LO′−IF andallows the component LO′+IF to pass to an amplifier 22 of selected fixedor variable gain, from which the RF signal is output at output 24. Ifthe originally recorded signal was an “image” signal, the componentLO′+IF is filtered out and the component LO′−IF is passed instead.

Hence, if the digital audio data recorded by the recorder 12 in FIG. 1is played back by the player 12′ of FIG. 3, the characteristics of theRF signal input on the line 2 are reproduced accurately on the output 24of the amplifier 22, and can be applied as a test signal to a receiverto be tested. The reproduced RF signal may be shifted in frequencyrelative to the recorded RF signal by choosing the frequency LO′ to bedifferent from LO.

Thus, instead of transporting test transmitter equipment to the receiverto be tested, one need only transport a digital audio tape or compactdisc, or send a data file to the test site, where the test signal can bereproduced using simple and inexpensive up-converting equipment and afilter. The use of standard digital audio recording media isparticularly advantageous, since recording and play back equipment forsuch media are commonly available. However, if such equipment is notavailable at the test side, a portable DAT or CD player or a portablecomputer can be easily transported to the test site.

Moreover, the RF test signal can be reproduced without broadcasting itover the air, thus avoiding the risk of eavesdropping by third partiesand ensuring that a test signal having the required characteristics isapplied to the test equipment without further degradation caused bybroadcasting the test signal over the air.

A third embodiment of the apparatus shown in FIG. 3 will now bedescribed with reference to FIG. 4. The apparatus to be tested comprisesan RF demodulator 26, connected via a suitable connector to the output24, which demodulates the RF signal to produce data which is input to avoice codec 28 so as to produce a voice signal which is output by aspeaker 30.

In this example, the RF signal comprises test data for testing theperformance of the codec 28. The radio frequency signal on the line 24is a reproduction of an RF signal input on the line 2 in the apparatusof FIG. 1, the RF signal being generated for example by an Inmarsat-M™modem.

A fourth advantageous embodiment of the present invention will now bedescribed with reference to FIG. 5, in which a reproduced test signal isapplied to a transceiver 34 and the test signal and the response of thetransceiver 34 are recorded on separate channels for later analysis.

The reproduced audio frequency test signal IF is output from the player12′ to an up-conversion circuit 32, comprising or example the mixer 16,local oscillator 18, bandpass filter 20 and amplifier 22 of theembodiment of FIG. 3. The RF-converted test signal from the output 24 isinput to the transceiver 34 under test, which may for example be aprototype Inmarsat-M™ modem. In this example, the RF test signalcontains the messages normally sent by a transmitter engaged in a callset-up protocol. The transceiver 34 transmits response signals RF′,comprising responses to the call set-up messages. These response signalsRF′ are down-converted by a down-conversion circuit 36, which may forexample comprise the mixer 4, local oscillator 6, low-pass filter 8 andamplifier 10 of the embodiment in shown in FIG. 1.

“The down-converted output signal RF′−LO of the down-conversion circuit36 is input to one channel input 12 of the digital audio recorder 12through a suitable connector. The audio frequency test signals IF outputby the player 12′ are connected via an audio signal connector to anotherinput channel I₁ of the recorder 12. Almost all digital audio recorders,including audio processing cards for computers, are able to recordstereo channels and therefore the input channels I₁ and I₂ mayadvantageously be the left and right stereo inputs of the recorder 12.Subsequently, the recordings of either of the channels I₁ and I₂ can beanalyzed to determined whether the transceiver 34 complies with thedesired standards, for example by outputting either of the channeloutputs to an apparatus as in the embodiment in FIG. 3 or by analyzingdirectly the audio frequency outputs of either of the channels on asuitably programmed computer, so as to interpret the timing and type ofsignals output by the transceiver 34.”

A fifth advantageous embodiment of the present invention will now bedescribed with reference to FIG. 6. This is used to record unidentifiedor interfering signals, for later analysis.

RF signals are received by an antenna 38 and input to a spectrumanalyser 40, which displays the power spectrum of an RF band. Anoperator monitors the spectrum analyser 40 and detects the occurrence ofan unexpected signal. The RF signals are input to apparatus similar tothat of FIG. 1. The local oscillator 6 is tunable by the operator toselect a wave band containing the signal of interest. The signal isrecorded on the digital audio recorder 12 for later analysis.

For example, if an error has occurred in the frequency allocation of achannel, the apparatus shown in FIG. 6 may be used to record theincorrectly allocated channel, which may later be analysed to determinethe identity of the user to whom the channel was allocated. It may thenbe determined whether the error was caused by the user or by anincorrect allocation by the system.

The apparatus shown in FIG. 6 may be automated so that detection of anRF signal having predefined characteristics by the spectrum analyser 40initiates recording by the digital audio recorder 12. For example, thespectrum analyser 40 is controlled by a computer, which receivesspectrum analysis data from the spectrum analyser 40. The recorder 12 isimplemented as an audio card on the computer and the computer beginsstoring digital audio data from the audio card if the output of thespectrum analyser 40 satisfies a predetermined criterion. The localoscillator 6 is also tunable under the control of the computer. In thisway, the computer can control the spectrum analyser 40 to sweep througha wide band of the radio frequency spectrum and the local oscillator maybe set so that signals of interest can be recorded as they are detected.

Alternatively, the spectrum analyser 40 may be dispensed with altogetherand the computer may itself analyse the output RF−LO so as to determinewhether unexpected signals are present. The digital audio data output bythe audio card may be buffered, so that, if it is determined that anunexpected signal is present in the buffered data, the buffered data isrecorded. Thus, none of the unexpected signal is lost due to the delaybetween detecting the unexpected signal and initiating recording by therecorder 12.

A sixth embodiment, including the embodiment shown in FIG. 3 will now bedescribed with reference to FIG. 7. In this embodiment, the embodimentof FIG. 3 is used to provide an RF signal for an operator trainingsession to give trainee operators practice in demodulating or analysingsignals. The RF signal on the output 24 is connected to a splitter 42 toproduce multiple outputs of the same signal. Each of these multipleoutputs is connected by a suitable connector to one of a set ofreceivers 44 a to 44 f for a corresponding group of trainees, so thateach of the trainees can practice demodulating or analysing the samesignals.

A seventh embodiment, including the embodiment of FIG. 3 will now bedescribed with reference to FIG. 8, in which the embodiment is used tosupply an RF signal for a local radio transmitter, including anamplifier 44 and an antenna 46.

In this way, a radio programme can be modulated into the form in whichit is to be transmitted and then recorded by the apparatus shown in FIG.1. The digital audio medium can then be distributed to local radiostations and reproduced using the apparatus shown in FIG. 8. Thisembodiment provides an economical way of distributing programmematerial.

With the advent of digital radio broadcasting, it is thought to benecessary to upgrade local radio transmitters to include advanceddigital modulation equipment. However, using the apparatus shown in FIG.8, the radio programme can be digitally modulated and recorded by adistributor and the local radio station only needs the apparatus shownin FIG. 8 to reproduce the digitally modulated signal, without having toinstall digital modulation equipment. This is particularly advantageousif numerous incompatible digital radio formats exist, which wouldotherwise require the local radio station to include numerous sets ordigital modulation equipment. The broadcast frequency may be varied bycontrolling the frequency LO′.

The above embodiments are given purely by way of example and the presentinvention is not limited thereto. Other digital audio recording,distribution and playback techniques may be used and it is expected thatnew methods providing greater storage capacity and/or fidelity ofrecording and playback will be developed. Such variants are not excludedfrom the scope of the present invention.

The present invention is not limited to use with modulated signals ofany specific system, such as one of the Inmarsat™ systems, but may beapplied to any situation in which it is desired to record and/orreproduce modulated signals while preserving their characteristics. Therecorded and/or reproduced signals may be RF signals. Alternatively,modulated test signals may be output by test modems in the audiofrequency range and therefore may be recorded without down conversion.Such signals may be reproduced in the audio frequency range for use withtest equipment, or may be up converted into a radio frequency band.

What is claimed is:
 1. Transceiver testing apparatus, comprising: afirst storage means storing first digital data representing a firstmodulated signal for output to a transceiver, a digital-to-analogconverter arranged to receive said first digital data from said firststorage means and to convert said first digital data to generate saidfirst modulated signal in an audio frequency range, wherein amplitude,phase and frequency information recorded in said first digital data arereproduced in said first modulated signal; the testing apparatus furthercomprising: means for outputting a second modulated signal from thetransceiver in the audio frequency range; an analog-to-digital converterarranged to receive said second modulated signal and to convert thesecond modulated signal to second digital data; and a second storagemeans arranged to store said second digital data such that amplitude,phase and frequency characteristics of the second modulated signal arerecorded in said stored second digital data; and a connector arranged tosupply the first modulated signal output by the digital-to-analogconverter to an analog input of said analog-to-digital converter. 2.Apparatus as claimed in claim 1, wherein said analog to digitalconverter has first and second channel inputs, the first channel inputbeing connected to said connector and the second channel input beingarranged to receive the second modulated signal output by thetransceiver.
 3. Apparatus as claimed in claim 2, wherein the first andsecond channel inputs comprise stereo audio inputs.
 4. Apparatus asclaimed in claim 1 or 2, wherein said means for outputting said secondmodulated signal comprises down-converting means for down-converting infrequency a modulated radio frequency signal from said transceiver togenerate said second modulated signal in the audio frequency range. 5.Apparatus as claimed in claim 1 or 2, including an amplifier arranged toamplify the output second modulated signal for input to theanalog-to-digital converter.
 6. Apparatus as claimed in claim 1 or 2,including up-converting means for up-converting the first modulatedsignal from said audio frequency range to a radio frequency band. 7.Apparatus as claimed in claim 6, including an amplifier for amplifyingsaid first modulated signal in the radio frequency band.
 8. Apparatus asclaimed in claim 1, wherein the first and/or second storage meanscomprises digital tape storage equipment.
 9. Apparatus as claimed inclaim 1, wherein the first and/or second storage means comprises opticaldisc storage equipment.
 10. Apparatus as claimed in claim 1 wherein thefirst and/or second storage means comprises an electronic memory. 11.Apparatus as claimed in claim 1, wherein the first and/or second storagemeans comprises magnetic disc storage equipment.